1. Field of the Invention
The present invention relates to novel linear polychlorophosphazenes and to the process for their preparation.
2. Description of the Prior Art
Numerous publications describe the preparation of cyclic polychlorophosphazenes. Such preparation is relatively easy to accomplish in view of the tendency of the lower compounds of the series of the chlorophosphazenes to ring formation. It is noted, however, that linear polychlorophosphazenes have a considerably greater economic interest than cyclic polychlorophosphazenes, on account of the range of their potential uses as materials having very extensive and very diverse applications like those of silicones, plastics materials and natural or synthetic elastomers, as antiflame and anticombustion materials or additives conferring antiflame and anticombustion properties on the materials and substances to which they are added, as coatings, in particular sealing coatings, in the biomedical field, as fertilizers, or as lubricants, in particular. Moreover, certain elastomers obtained by various substitutions on linear polychlorophosphazenes are found to have excellent behavior at low temperatures, as well as with respect to corrosive reactants. The major application of polychlorophosphazenes is represented by the fact that the latter constitute the optimum starting point for the production of polyorganophosphazenes, which are polymers having remarkable properties.
Some processes for the preparation of linear polychlorophosphazenes have been proposed in accordance with the prior art. Thus L. G. LUND, N. L. PADDOCK, F. E. PROCTOR and H. T. SEARLE, J. Chem. Soc. London, p. 2542, 1960, have described the production of polychlorophosphazenes according to the following reaction diagram: ##STR2## in symmetrical tetrachloroethane as solvent. However this route involves a large number of reaction steps and constitutes a long and laborious process, since the crude product to which it gives rise is a mixture of cyclic compounds, present in the proportion of 90% and linear compounds of the PCl.sub.5 (PNCl.sub.2).sub.n type where n does not exceed 20, in a proportion of 10%.
To obtain longer linear polymers, the cyclic compounds are then treated with solvents to separate the trimer and tetramer from which there is extracted, by means of suitable solvents, the pure (NPCl.sub.2).sub.3 which is subjected to heat polymerization under reduced pressure at a temperature of 250.degree. C. for two days, to give a linear (PNCl.sub.2).sub.n polymer with an optimum conversion ratio of 70% ((H. R. ALLCOCK, R. L. KUGEL, K. J. VALAN - Inorg. Chem. 1966, 5, 1709).
On the other hand, BECKE-GOEHRING and LEHR Z. Anorg. all. Chem. 1963, 325, p. 287, have prepared dichlorophosphorylpentachlorodiphosphazene Cl.sub.2 (O)P--N.dbd.PCl.sub.2 --.sub.2 Cl by the reaction of SO.sub.2 on the ionic compound (Cl.sub.3 P.dbd.N--PCl.sub.2 .dbd.N--PCl.sub.3)PCl.sub.6 obtained by the reaction of phosphorus pentachloride with ammonium chloride in a solvent with a low dielectric constant: it was not however possible for these authors to obtain higher homologs in which n&gt;2, by reason of the formation of cyclic polychlorophosphazenes consequent upon the reaction of the ionic compound with the ammonium chloride. KIREJEW and Coll., Z. obsc. Chim. URSS, 42 (104), 510, (1972) have, besides, described the preparation of dichlorophosphorylheptachlorotriphosphazene Cl.sub.2 (O)P----N.dbd.PCl.sub.2 --.sub.3 Cl by cleavage of the ring of the hexachlorocyclotriphosphazene by means of PCl.sub.5 and reaction of the scission product with SO.sub.2. On the other hand, RIESEL and SOMIESKI, Z. anorg. allg. Chem. 411, p. 148-152, (1975) proposed the synthesis of the first members of the series of the linear phosphorylchlorophosphazenes by step by step lengthening of the P--N--P chain by successively reacting POCl.sub.3 or short chain phosphorylchlorophosphazenes, with hexamethyldisilazane and phosphorus pentachloride. However, they did not succeed in going beyond the triphosphazene of the formula Cl.sub.2 (O)P----N.dbd.PCl.sub.2 ----.sub.3 Cl which, all the same, is an unstable compound which is decomposed by heating above 100.degree. C., with the liberation of POCl.sub.3 and the formation of PNCl.sub.2 n high polymers and of about 20% of hexachlorocyclotriphosphazene, the diphosphazene having besides also a tendency, if it is overheated, to become condensed with the liberation of POCl.sub.3 and formation of PNCl.sub.2 n high polymers.
It is also known from the USSR Pat. No. 176 416 in the names of V. S. Yakubovich, I. V. Lebedova, A. Ya. Yakubovich and N. I. Shvetsov, Karpov Scientific Research Physical-Chemical Institute, filed 25 March 1963 and granted 22 November 1965, that polychlorophosphazenes of the formula Cl.sub.2 (O)P NPCl.sub.2 n Cl can be prepared from the following polymers: Cl(PNCl.sub.2).sub.10 PCl.sub.4 and PCl.sub.5 (NPCl.sub.2).sub.4, by reaction with SO.sub.2, which signifies that the starting product is always a low polymer, and in no case the monomer. It is impossible to obtain the monomer by the process of this USSR Pat., as this process teaches that the starting product is obtained from a mixture of cyclic and linear compounds which are the reaction product of PCl.sub.5 and NH.sub.4 Cl.
It emerges from the Prior Art mentioned in the foregoing, that the various direct routes proposed do not enable the production of linear polychlorophosphazenes, and only permit, at the best, the obtaining of linear dimers or trimers whose potential industrial applications are very limited. The sole route at present adopted for the production of long chain linear polychlorophosphazenes, is the indirect route, described first above, of the treatment of cyclic compounds. However, the large number of operations that it requires, of which the yield is rarely quantitative, and the utilization of large amounts of expensive solvents, make its cost price prohibitive and weighs heavily on the cost of the polychlorophosphazenes obtained. In addition, this method only permits the obtaining of very long chain polychlorophosphazenes, of the order of approximately 15,000 NPCl.sub.2 units, and it cannot be controlled to permit the production of polychlorophosphazenes whose chain length can be determined at will and notably of short or medium chain length linear polychlorophosphazenes, that is to say, including from 4 to 1,000 units or more, for example.
Consequently, it is an object of the present invention to provide a novel process for the preparation of polychlorophosphazenes which responds better to the requirements of practice than the processes proposed by the Prior Art, notably in that it constitutes a simple, rapid and quantitative synthetic process which enables the cost price of the products obtained to be considerably lowered. It is another object of the invention to enable the production of polychlorophosphazenes by a direct route, that is to say without resorting to transformation of cyclic polychlorophosphazenes. It is a further object of the invention to enable the production of polymers in which it is possible to control at will the degree of polymerization according to the applications envisaged.